Patent Document 1 (shown below) shows a signal processing apparatus which performs a frequency spectrum analysis of a signal output from a knock sensor mounted on an internal combustion engine, using the discrete Fourier transform algorithm (hereinafter referred to as “DFT algorithm”) and the fast Fourier transform algorithm (hereinafter referred to as “FFT algorithm”). The intensity of a necessary frequency component (which is specifically a central frequency component of the knock sensor output) may not be obtained by the FFT algorithm since a special algorithm is employed in the FFT algorithm for improving a calculation speed. Therefore, in the above signal processing apparatus, the central frequency component of the knock sensor is calculated using the DFT algorithm, and frequency components other than the central frequency component are calculated using the FFT algorithm.